Keeping Texture2D object(s) for interpolation HLSL - hlsl

Question

Recently someone added a module to OBS Studio which lets anyone incorporate their own shaders into OBS. I've never touched on writing shaders, but after reading some material, I get the gist, it's a function that returns a bit of memory representing the RGBA values of a specific pixel.
Here's the issue, I'm too new to this, it looks to me like there's a few different high level shader languages? I have no clue which one OBS Studio is using, the author of https://github.com/nleseul/obs-shaderfilter doesn't seem to know either. Any pointers to what syntax / what documentation would be of course greatly appreciated.
What I'm aiming to do is a very dumbed down motion blur. Namely though my goal is that I'd like to keep a few frames in a buffer of some sort to work with, I figure that'd be a pretty useful thing to do for other effects...and that's where I'm stuck. Here's what I've got following from Shaders for Game Programmers and Artists pg.87 adapted to work w/ the shader plugin*
uniform float4 blur_factor;
texture2d previmage;
//texture2d image; //this is the input data we're playing around with
float4 mainImage(VertData v_in) : TARGET
{
float4 originalFrame = image.Sample(textureSampler, v_in.uv);
float4 oldFrame = previmage.Sample(textureSampler, v_in.uv);
//this function's going pixel by pixel, I should be pushing
//to the frame buffer here* or there should be some way of modifying
//the frame buffer pixel by pixel, second option makes more sense to do
if(v_in.uv.x == 1 && v_in.uv.y == 1){
//it couldn't have been this easy...it's never this easy
//uncommenting the line below clearly causes a problem, I don't have a debugger for this
//previmage = image;
//this doesn't work either, wishful thinking
//previmage = texture2d(image);
}
//this may not actually be the function to use for a motion blur but at the very least it's mixing two textures together so that I'd have a proof of concept working*
return lerp(originalFrame,oldFrame,blur_factor);
}

Answer 1

So, fortunately it turns out that when a shader fails the error is actually output to OBS's logs. I had a gut feeling that something was up w/ declaring variables globally. For the initial code I wrote I got this:
10:53:49.413: C:\Program Files (x86)\obs-studio\bin\64bit\ (Pixel shader, technique Draw, pass 0)(38,40-50): error X3004: undeclared identifier 'blur_factor'
10:53:49.413: C:\Program Files (x86)\obs-studio\bin\64bit\ (Pixel shader, technique Draw, pass 0)(38,12-51): error X3013: 'lerp': no matching 3 parameter intrinsic function
10:53:49.413: C:\Program Files (x86)\obs-studio\bin\64bit\ (Pixel shader, technique Draw, pass 0)(38,12-51): error X3013: Possible intrinsic functions are:
10:53:49.413: C:\Program Files (x86)\obs-studio\bin\64bit\ (Pixel shader, technique Draw, pass 0)(38,12-51): error X3013: lerp(float|half|min10float|min16float, float|half|min10float|min16float, float|half|min10float|min16float)
So in short...lerp was in fact NOT what I was looking to use, didn't properly call it with the right data types to begin with. So after tossing that I decided to write the shader assuming I could make an array of texture2d's ...
texture2d previmage[8];
float4 mainImage(VertData v_in) : TARGET{
//...etc etc
//manipulating the array, copying over the texture2d OBS provides
}
Then I get this:
11:12:46.880: C:\Program Files (x86)\obs-studio\bin\64bit\ (Pixel shader, technique Draw, pass 0)(27,3-31): error X3025: global variables are implicitly constant, enable compatibility mode to allow modification
Oh...so...global variables in HLSL have a weird behavior that I wouldn't know about, and it makes this approach a bit mute...can't modify constant values, and I need some sort of frame buffer.
Potential solution? Static Variables!
float4 mainImage(VertData v_in) : TARGET
{
int i = 0;
static texture2d previmage[8] = {image,image,image,image,image,image,image,image};
if(v_in.uv.x == 1 && v_in.uv.y == 1){
for(i = 0; i <= 6; i++){
previmage[i+1] = previmage[i];
}
previmage[0] = image;
}
float4 sum = 0;
float4 samples[8];
for(i = 0; i < 8; i++){
samples[i] = previmage[i].Sample(textureSampler, v_in.uv);
sum += samples[i];
}
return sum / 8.0;
}
Now...well at least the thing isn't throwing errors and it's rendering to the screen, unfortunately...I don't / can't see the effect...there's potentially a semantic error in here* or...maybe 8 frames isn't enough to really make for a noticeable motion blur.